1. Field of the Invention
The present invention relates to speakers and speaker systems, and in particular to a circuit and method for protecting speaker systems from overload while permitting substantially uniform speaker response.
2. The Prior Art
Sounds which are audible to the human ear lie within a broad frequency band ranging from approximately 20 Hz to about 20 kHz. It is desirable that speaker systems be capable of reproducing sounds at all of these audible frequencies.
Because this audio range covers so many octaves, more than one sound radiator is typically required for the best combination of efficiency, response smoothness, and broad directivity. By dividing the frequency range into parts, and assigning each part a suitable radiator, a superior acoustical result may be provided. Cross-over circuits are commonly used in these multiple speaker systems for accomplishing the frequency range division and for providing selected portions of the frequency range to appropriate speakers.
Cross-over circuits generally involve some type of filter arrangement wherein the audio range input signal is processed to provide several output signals of different frequency ranges, each frequency range being compatible with the reproduction capabilities of a speaker to which the output signal is transmitted. The frequency at which the different frequency ranges intersect is called the cross-over frequency. In prior art systems, this cross-over frequency remains a fixed value under all operating conditions, so that the frequency ranges which are transmitted to the various speakers remain unchanged.
In systems with a single power amplifier, so called "passive" cross-over circuits are connected between the power amplifier and the individual speakers. These passive devices comprise relatively large coils and capacitors which function to divide the audio spectrum into frequency bands at high signal levels.
In more elaborate systems, such as those producing high power output signals, several power amplifiers may be utilized. In these types of systems, "electronic" cross-over circuits split the frequency spectrum at low signal levels prior to transmission to the individual power amplifiers, which are each directly connected to a speaker.
Although the problem of handling sound reproduction across the full audible frequency range is minimized by use of multiple speaker systems, significant limitations in output signal quality are caused by the speakers and speaker systems themselves. Specifically, the output response of an individual speaker typically tends to be somewhat uniform within a given frequency range, which may be quite narrow depending upon the speaker. However, outside of this frequency range the speaker response may deteriorate at a rapid rate.
This deterioration is evidenced by a significant reduction in gain as the output signal frequency becomes more distant from the given frequency range. This problem is experienced more often on the high and low frequency ends of the audible range, and is generally less apparent in the mid-frequency range. This problem is most often overcome through use of equalizers which serve the function of lifting the low and high end of the audible spectrum by compensating for the reduced gain at those frequencies. Thus, with proper use of equalizers, a substantially uniform speaker response over most of the audible frequency range is possible.
Compensation of the reduction in gain by use of equalizers or similar devices tends to overcome the problem of obtaining uniform speaker response, but functions to create other problems which could ultimately result in physical damage to the speaker system itself. Specifically, structural limitations in speaker operation are generally first reached at high and low frequencies. As signal power is increased at high frequencies, rapid vibrations of the diaphragm coupled with the increased power produce excessive heat dissipation in the voice coil. The use of excess power at the high frequency level will ultimately result in the melting of solder connections. Thus, the amount of power which may be added to a system without creating thermal overload is more limited when the system also includes high frequency gain compensation.
At the lower frequencies, the allowable input power is limited by the finite excursion capabilities of the speaker cone or diaphragm. The excursion of this speaker membrane is inversely and linearly proportional to the frequency. For example, if a particular speaker cone moves .+-.0.1 inch for a given power input at 1,000 Hz, then with the same power input applied at 500 Hz, the cone would move .+-.0.2 inches, and at 250 Hz it would move .+-.0.4 inches. Since every speaker is limited at some point in its excursion ability, it becomes apparent that speakers may be destroyed at low frequencies with only a fraction of the power that they handle at the higher frequencies.
Overheating of speakers at high frequencies is often prevented by devices such as current limiters, compressors, fuses and heat sensitive resistors, all of which are commercially available and readily adaptable for high frequency protection. However, these types of devices cannot be used to provide low frequency protection for speakers. For example, a simple fuse connected in series with the speaker can prevent excessive current from damaging the speaker by preventing flow of current beyond a given level. As was pointed out above, speakers can operate at high frequencies at significantly greater power levels than is allowable at the lower frequencies. Thus, limiting the current flow to the speaker at a safe level for low frequency operation significantly and unnecessarily restricts the high frequency operation of the speaker. For these reasons, the other devices described above are also not desirable for use in providing low frequency protection.
Of course, one approach for preventing damage to the high frequency speaker would be to simply set the crossover point at a frequency which is sufficiently high to prevent excessive excursion of the speaker diaphragm. It will be readily appreciated that this approach will simply result in poor performance by limiting the range of frequency reproduction by the speakers, or it will require the further expense of including an additional speaker for reproducing sounds in the frequency range below the crossover point.
Another approach which has been utilized in preventing speaker damage due to excessive diaphragm excursion is to utilize a cone driver or multiple compression driver which allows a high level of diaphragm excursion. In these cases, a low crossover point may be fixed without fear of damage to the speaker at high power levels. However, these types of systems are necessarily much more expensive than a single compression driver, and the cone drivers have a greater amount of distortion than a comparable, single compression driver. Of course, a single compression driver would be susceptible to damage due to excessive excursion, and thus, without more, the single compression driver is limited to use in a low power system which will not produce excessive excursion of the diaphragm at frequencies above the crossover point.
Another method which has been used to provide speaker protection in both low and high frequency overload conditions involves sensing unacceptable power levels and reducing the gain of the speaker in response. Although this method does provide speaker protection, it also does so at the cost of system performance. For example, upon sensing a high frequency thermal operating limiting, a reduction in output signal gain overcomes the problem but also unnecessarily reduces the gain at other, lower frequencies and thus degrades system performance across the full range of frequencies provided to the protected speaker. On the other hand, by reducing the gain to prevent the speaker from exceeding low frequency speaker excursion limits, high frequency operation is again adversely influenced in substantially the same manner as if a fuse were used to limit current flow, as described above. Thus, this protection method also does not provide for uniformity of system response, and it unnecessarily degrades overall system performance.
Still another method for avoiding damage due to excessive power is to incorporate several speakers of the same frequency range in parallel configuration. In this manner, the parallel speakers share the influence of increased power, thus reducing the likelihood of speaker failure and increasing the power handling capabilities as more parallel speakers are included in the system. Of course, this alternative is very costly due to the duplication of systems, and requires a substantial increase in the space required for housing and positioning of the speakers, as well as increasing the difficulty of moving the speakers between performance locations.
Accordingly, what is needed is a circuit and method for protecting speakers and speaker systems from damage due to excessive power levels at low frequencies, while still providing for substantially uniform system operation over a wide frequency range, improved system reliability, and a minimum number of speakers for producing a given audible sound reproduction.